The present invention pertains to apparatus and methods used for measuring surface shapes (surface topography) with high accuracy and precision. More specifically, the invention concerns such apparatus and methods that employ interferometry for measuring topographical characteristics of a surface of a sample such as an optical element.
A conventional use of a Fizeau interferometer or a Twyman-Green interferometer is the measurement of the shape (i.e., surface topography) of a spherical surface, such as the surface of a spherical lens. To perform such measurements using either of these types of interferometers a reference surface is conventionally required. I.e., conventional measurements of the surface topography of a spherical surface of a sample are determined by comparison with an actual corresponding xe2x80x9cidealxe2x80x9d reference surface. As a result, the accuracy of the measurements cannot exceed the accuracy of the reference surface.
A conventional way in which to solve such a problem is disclosed in Japanese Laid-Open (Kokai) Patent Application No. 2-228505 disclosing an interferometer not requiring a reference surface. Specifically, this reference discloses a so-called point-diffraction interferometer, abbreviated xe2x80x9cPDI.xe2x80x9d A PDI employs an ideal spherical-surface wave, generated by diffraction of light passing through a pinhole, for use as a reference wavefront. Such a device allows high-accuracy and high-precision measurements of the topography of aspherical surface.
Unfortunately, the conventional PDI technique summarized above is not usable for measuring the surface topography of an aspherical surface. This is because, when measuring a spherical surface, very few interference fringes (i.e., a xe2x80x9csparsexe2x80x9d pattern of fringes) are generated by regions of the surface where the curvature radius of the spherical wave produced by the pinhole coincides with the curvature radius of the sample. But, when measuring an aspherical surface, the curvature radius varies according to the location on the surface; as a result, the spacing of interference fringes is sparse only in regions that are too dense for measuring.
In view of the shortcomings of the conventional art as summarized above, an object of the invention is to provide apparatus and methods for performing high-accuracy measurements of the surface topography of aspherical surfaces as well as spherical surfaces.
To such end, and according to a first aspect of the invention, various representative embodiments of apparatus for measuring the surface topography of a test surface of a sample are provided. A first representative embodiment of such an apparatus comprises a point light source configured and situated relative to a detector and the sample so as to produce, from an input light, a beam of light divergently propagating as a prescribed wavefront from a point on the point light source. The beam comprises a measurement-beam portion, directed toward the test surface so as to reflect from the test surface, and a reference-beam portion. The light detector is configured to produce an output signal encoding data corresponding to an interference characteristic of light received by the detector. The point light source also comprises a reflective surface oriented so as to receive the measurement-beam portion reflected from the test surface and to cause the measurement-beam portion returning to the point light source to reflect from the reflective surface toward the detector. The measurement-beam portion reflected from the test surface and the reference-beam portion interfere with each other so as to produce an interference fringe received by the detector. The interference fringe has a characteristic corresponding to a surface topography of the test surface relative to the prescribed wavefront. An actuator is configured and situated so as to move at least one of the sample and the point light source relative to each other so as to change the distance between the test surface and the point light source as required. A processor is situated so as to receive the output signal from the detector. The processor is configured to provide a measurement of the surface topography from the interference fringe received by the detector.
The point light source desirably comprises a reflective mirror defining a pinhole, wherein the prescribed wavefront is produced by diffraction of the input light as the input light passes through the pinhole. The reflective mirror can be oriented so as to define a plane that is perpendicular to a propagation axis of the input light incident on the point light source. Alternatively, the reflective mirror can be oriented so as to define a plane that is at an angle of less than 90xc2x0 to a propagation axis of the input light incident on the point light source.
In an alternative configuration, the point light source can comprise an optical fiber configured to conduct the input light. Such an optical fiber desirably comprises an end face serving as the reflective surface of the point light source. The end face also serves as the point from which the prescribed wavefront divergently propagates due to diffraction of the input light.
The prescribed wavefront can be a spherical wavefront. Alternatively, the prescribed wavefront can be any of various suitable aspherical wavefronts.
The point light source can be configured and oriented such that the measurement beam interferes with the reference beam as the measurement beam, reflected from the test surface, propagates to the reflective surface of the point light source. Alternatively, the point light source can be configured and oriented such that the measurement beam interferes with the reference beam as the measurement beam, reflected from the reflective surface of the point light source, propagates to the detector.
The apparatus can further comprise a light source configured to produce the input light. The input light can comprise a single or multiple wavelengths of light as required. If the input light comprises multiple wavelengths, a wavelength selector can be included to permit selection, from the multiple wavelengths of input light, a particular wavelength for input to the point light source. In such a configuration, the point light source can be configured to produce, from the particular wavelength, the measurement beam portion and the reference beam portion.
The apparatus can further comprise a light-path adjuster situated between the input light source and the point light source. The light-path adjuster is desirably configured to cause a path length of the reference-beam portion to coincide with a path length of the measurement-beam portion so as to permit adjustment of a contrast parameter of the interference fringe.
According to another aspect of the invention, methods are provided for measuring a profile of a test surface of a sample. According to a representative embodiment of such a method, a point light source is provided having a reflective surface. The point light source is situated so as to receive an input light and to produce from the input light a measurement-beam portion and a reference-beam portion divergently propagating as a prescribed wavefront from a point. The test surface is irradiated with the measurement-beam portion so as to cause the measurement-beam portion to reflect from the test surface and then reflect from the reflective surface. At a first distance of the test surface from the point light source, the measurement-beam portion reflected from the test surface interferes with the reference-beam portion so as to produce a first pattern of interference fringes. A pattern characteristic of the first pattern of interference fringes is detected. The first distance of the test surface from the point light source is changed to a second distance. At the second distance, the measurement-beam portion reflected from the test surface interferes with the reference-beam portion so as to produce a second pattern of interference fringes. A pattern characteristic of the second pattern of interference fringes is detected, and the pattern characteristic of the second pattern of interference fringes is compared with the pattern characteristic of the first pattern of interference fringes so as to obtain a measurement of a topographical profile of the test surface.
In the above-summarized method, the reflective surface of the point light source can define a pinhole. In such a configuration, the prescribed wavefront of the measurement-beam and reference-beam portions is generated by diffraction of input light as the input light passes through the pinhole. Alternatively, the point light source can be configured as an optical fiber that conducts input light, wherein the optical fiber terminates with an end face defining the reflective surface of the point light source. In this alternative configuration, the prescribed wavefront of the measurement-beam and reference-beam portions is generated by diffraction of input light as the input light is conducted through the optical fiber and exits the end face.
The input light can comprise multiple wavelengths. In such an instance, a first specific wavelength, of the multiple wavelengths, of input light is selected to be received by the point light source. The point light source generates the measurement-beam and reference-beam portions from the first specific wavelength. Measurements can be performed with the measurement-beam portion and reference-beam portion at the first specific wavelength. Then, a second specific wavelength, of the multiple wavelengths, of input light is selected to be received by the point light source. The point light source generates the measurement-beam and reference-beam portions from the second specific wavelength with which measurements are again obtained. The pattern characteristics of all sets of interference fringes are compared so as to obtain a measurement of a topographical profile of the test surface.
Desirably the interfering measurement-beam and reference-beam portions are collimated as said portions are directed to the light-receiving surface of the image detector. Such collimation can be performed by passage of the portions through a lens.
If the input light comprises a temporally incoherent light, the method can further include providing an input-light source for producing the input light supplied to the point light source. After removing the sample from a measurement position, a light-path distance from the input-light source to the point light source is adjusted until interference fringes are detected having maximal contrast. The sample is then returned to the measurement position and the light-path distance from the input-light source and the point light source is adjusted until interference fringes are detected having maximal contrast. A difference in the light-path distance from the input-light source and the point light source is determined when the sample is in the measurement position versus when the sample is not in the measurement position. The difference provides a measure of the distance from the point light source to the test surface.
The present invention also permits measurements of the topographical profile of an entire aspherical surface at one time. A representative apparatus for such a purpose comprises a light detector configured to produce an output signal encoding data corresponding to an interference characteristic of light received by the detector. A point light source is configured and situated relative to the detector and the sample so as to produce, from an input light, a beam of light divergently propagating as a prescribed spherical wavefront from a point on the point light source. The beam comprises a measurement-beam portion directed toward the test surface so as to reflect from the test surface, and a reference-beam portion. The point light source comprises a reflective surface oriented so as to receive the measurement-beam portion reflected from the test surface and to cause the measurement-beam portion returning to the point light source to reflect from the reflective surface toward the detector. The measurement-beam portion reflected from the test surface and the reference-beam portion interfere with each other so as to produce an interference fringe received by the detector. The interference fringe has a characteristic corresponding to a surface topography of the test surface relative to the prescribed wavefront. A processor is situated so as to receive the output signal from the detector, and is configured to determine a measurement of the surface topography from the interference fringe received by the detector. An optical element is situated between the point light source and the test surface. The optical element is configured to convert the spherical wavefront of the measurement light, propagating from the point to the test surface, into a desired aspherical wavefront.
Another embodiment of an apparatus for measuring a profile of an aspherical test surface of a sample comprises a point light source. The point light source is configured and situated relative to the sample so as to produce, from an input light, a measurement beam of light propagating as a prescribed spherical wavefront from a point on the point light source to the test surface. A beamsplitter is situated so as to receive light of the measurement beam reflected from the test surface. The beamsplitter is configured to split the reflected measurement beam into first and second measurement-beam portions each propagating along a respective path. A light-diffraction element is situated in the path of the first measurement-beam portion. The light-diffraction element is configured to diffract the light of the first measurement-beam portion into multiple orders of diffracted light. The light-diffraction element is situated so as to cause the diffracted light to interfere with light of the second measurement-beam portion and produce interference fringes. A detector is configured and situated so as to detect the interference fringes. An optical element can be situated between the test surface and the beamsplitter and configured so as to convert the aspherical wavefront of the measurement beam reflected from the test surface into a desired spherical wavefront. Alternatively, the optical element can be situated between the point light source and the test surface and configured so as to convert the spherical wavefront of the measurement beam from the point light source into a desired aspherical wavefront.
Yet another embodiment of an apparatus for measuring a profile of an aspherical test surface of a sample includes a point light source as summarized above that produces a reference beam and a measurement beam. Light of the measurement beam reflected from the test surface interferes with the reference beam to produce interference fringes. A detector receives the interference fringes and a processor connected to the detector analyzes the interference fringes so as to calculate, from such analyses, a state of the interference fringes. A reflective surface is situated in a light path between a source of the input light and the test surface, wherein the reflective surface is configured to define the point light source. An optical element is situated in a light path different from the light path extending from the test surface to the reflective surface. The optical element converts the wavefront propagating from the test surface from having an aspherical wavefront to having a spherical wavefront. The measurement beam emitted from the point light source reflects from the test surface, then from the reflective surface, and passes through the optical element.
Alternatively, the optical element can be situated in a light path extending from the test surface to the reflective surface. Such an optical element converts the wavefront propagating from the test surface from having a spherical wavefront to having an aspherical wavefront. A light-diffraction element is situated in a light path different from the light path extending from the test surface to the reflective surface. The light-diffraction element defines a point light source and generates, from light entering the point light source, a diffracted light propagating from the point light source. In such a configuration, the measurement beam emitted from the point light source passes a first time through the optical element, reflects from the test surface, passes a second time through the optical element, and reflects from the reflective surface to the detector.
The foregoing and other features and advantages of the invention will be more apparent from the following detailed description, which proceeds with reference to the accompanying drawings.